Sorbent tubes have many different applications, and include a packing material that can be used for adsorption. One application for sorbent tubes includes gas chromatography. Gas chromatography is essentially a physical method of separation in which constituents of a test sample in a carrier gas are adsorbed or absorbed and then desorbed by a stationary phase material in a column. A pulse of the sample is introduced into a steady flow of carrier gas, which carries the sample into a chromatographic column. The inside of the column is lined with a liquid, and interactions between this liquid and the various components of the sample—which differ based upon differences among partition coefficients of the elements—cause the sample to be separated into the respective elements. At the end of the column, the individual components are more or less separated in time. Detection of the gas provides a time-scaled pattern, typically called a chromatogram, that, by calibration or comparison with known samples, indicates the constituents, and the specific concentrations thereof, which are present in the test sample. An example of the process by which this occurs is described in U.S. Pat. No. 5,545,252 to Hinshaw.
One common application of chromatographic analysis is the use of thermal desorption units to determine the constituents of a particular environment. For example, it is often desired to detect the amount of volatile organic compounds (VOCs) present in a certain sample of air. One way of doing this is by first transporting a tube packed with an adsorbent material into the environment to be tested, and allowing the VOCs in the air to migrate into the tube through natural diffusion, typically termed “diffusive” or “passive sampling.” Alternatively, the VOCs may be collected by drawing a sample of gas (typically ambient air) through such a tube using a small vacuum pump, commonly referred to as “pumped sampling.” In each case, the analytes to be measured (i.e., the VOCs) are retained by and concentrated on the adsorbent as the air passes through the tube. As is briefly described in U.S. Pat. No. 6,649,129 to Neal, once the VOCs are first collected in this fashion, and then, the tube is subsequently heated in a thermal desorption instrument, and a flow of inert gas, such as Helium or Nitrogen, is applied to the tube to sweep the VOCs out of the tube and into the chromatographic column for separation and analysis.
Regardless of what particular application is being used, it is also often desired to pre-concentrate the analytes in the sample, and occasionally, remove moisture therefrom, prior to introducing the sample into the chromatographic column. Accordingly, as disclosed in U.S. Pat. Nos. 5,792,423 and 6,395,560 to Markelov, these systems will typically include some kind of “trap” for this purpose, which retains the analytes as they are carried through the trap, and which are later released from the trap, usually by heating, and swept into the chromatographic column. One example is an adsorbent trap, which typically includes a tube packed with a suitable adsorbent material, which adsorbs the analytes as the sample gas first passes through the tube, and from which the analytes are subsequently desorbed into the chromatographic column, such as the arrangements disclosed in U.S. Pat. No. 5,932,482 to Markelov and U.S. Pat. No. 6,652,625 to Tipler. flow rate at a tube outlet. Although the illustrated embodiments include applications directed to gas chromatography systems and methods, it can be understood that the present teachings can be equally applied to other sorbent tube applications.
In an embodiment, the present teachings include methods that include providing a vessel having an adsorbent disposed therein, the vessel having an inlet and an outlet for communicating a carrier gas through the vessel, determining a differential pressure between the inlet and the outlet for a known flow rate at the outlet; and, determining a geometric measure for the vessel based on a ratio of the known flow rate and the differential pressure. In determining a differential pressure, the methods can include providing at least one sensor to provide measurements related to the differential pressure. Determining a geometric measure can also include factoring a viscosity of the carrier gas in the ratio. In embodiments, the vessel may include an adsorbent trap or a sample tube. The methods can further include one or more of determining a flow rate at ambient pressure using a factor based on a ratio of a gas pressure at the output and ambient pressure, and, determining a flow rate at ambient temperature using a factor based on a ratio of a temperature at the output and ambient temperature.
In some embodiments, the present teachings relate to a chromatographic method that includes providing a vessel in a carrier gas flow path to a chromatographic column, where the vessel includes a gas inlet, a gas outlet, and, an adsorbent disposed in the vessel. The chromatographic methods also include measuring a differential pressure along the length of the adsorbent, determining a flow rate at the outlet, and, determining a unit flow per unit pressure based on a ratio of the flow rate and the differential pressure. In some embodiments, the methods can include adjusting the unit flow per unit pressure based on a viscosity of the gas. In embodiments, the chromatographic methods include determining (i) a flow rate at ambient pressure using a factor based on a ratio of a gas pressure at the output and ambient pressure, and/or, (ii) a flow rate at ambient temperature using a factor based on a ratio of a temperature at the output and ambient temperature.
In an embodiment, the present teachings thus comprise a chromatographic system, including a carrier gas inlet for supplying carrier gas, a chromatographic column for receiving the carrier gas, a flow path through which the carrier gas is communicated from the carrier gas inlet to the chromatographic column, a vessel disposed in the flow path, the vessel having an adsorbent disposed therein, wherein the vessel includes an inlet and an outlet for communicating the carrier gas through the vessel, and, at least one sensor in communication with said flow path, the at least one sensor providing at least one
However, one problem that exists in these various systems is that the sorbent tubes serving as the traps, as well as the sorbent tube serving as initial sampling tubes when dealing with applications involving the traditional thermal desorption units discussed above, is that the integrity of these tubes is sometimes compromised. Several causes of this problem are illustrated in FIGS. 1A-B. For example, in order for thermal desorption to work successfully, the adsorbent 10 must be properly packed inside the tube 12. However, sometimes this does not occur, and voids 14 are formed in the adsorbent, as shown in FIG. 1A. These voids will channel some of the gas flow, thereby degrading the adsorption and desorption efficiency of the adsorbent packing.
Similarly, occasionally, the adsorbent becomes damaged as a result of improper packing or thermal shock, thereby producing small fragments (fines) 16 that occlude the interstices between the packing particles, as shown in FIG. 1B. As a result, the flow of gas is partially blocked during adsorption and desorption, again degrading the efficiency of the sorbent tube.